A latent electrostatic image is developed in a manner wherein, on such an image formed on a photosensitive member by one of various ways a toner which has been charged by one of various methods, for example, by friction with a carrier or a sleeve, is made to be statically adsorbed, and then the toner image is transferred onto a transfer sheet and fixed thereto for the fixation of the developed image.
The methods for fixation of developed images can be broadly divided into the following methods: so-called heat fixation method wherein toner is melted by application of heat and fusion-attached to paper; so-called pressure fixation method wherein toner particles are deformed by application of pressure and pressure-attached to paper; and so-called solvent fixation method wherein toner is melted by application of solvent or the like and melt attached to paper. The solvent fixation method, however, is not suited to practice because of the pollutive nature of the solvent.
At present the heat fixation method is in major use for the fixation. Copying machines based on the heat fixation method have made a remarkable progress through various improvements. But the fixing device is heated by an electric heater, and a copying machine requires time for heating up the fixing device before it is ready for the copying after switching it on. It is 10-15 seconds with a quick model and 1-5 minutes ordinarily that it takes for the preliminary heating before the operation. Furthermore, when a fixing device loses heat and its temperature lowers, it requires reheating up to a prescribed temperature. Most of the copying machines based on the heat fixation method, therefore, exceeds 1 KW in power consumption, a major part of the power being expended for heating the fixation device.
The pressure fixation method is advantageous, having no need of the preliminary heating which was mentioned above and not wasting electric energy for the heating. But the conventional pressure fixation method also involves defects in that the toner is fixed at such a high pressure that the pressure causes the transfer sheet to curl up and in that the equipment costs high for this mechanism. Furthermore, lower firmness of fixation, compared with the heat fixation method, constitutes another problem. It has been awaited, therefore, for a capsule toner to be commercially introduced, one improved in fixation property and capable of fixation by application of a low pressure (80-400 kg/cm.sup.2).
For the production of capsule toners, it has already been proposed to introduce, for example, a spray-dry method, interfacial polymerization method, coacervation method, phase-separation method, and in-situ polymerization method (references: U.S. Pat. Nos. 3,338,991, 3,326,848, 3,502,582 etc.).
A particle of capsule toner consists of a shell and a filling therein, said shell consisting mainly of a resin which is designed to be broken by pressure and said filling consisting mainly of coloring materials, such as carbon black and magnetic powder, and binders, such as a resin for fixation and a wax.
A capsule toner is known to be suited to a pressure fixation method primarily for the reasons that high pressure is not required for the fixation and that good fixation results, but the capsule toners known so far have not been quite satisfactory with respect to their properties essential to toner.
An example is a capsule toner of the so-called solid filling type, made public in U.S. Pat. No. 4,259,426, whose filling consists mainly of a solid wax. A capsule toner of the solid filling type is easy to be fixed without requiring high pressure for the fixation, but it produces picture images with unsatisfactory results, the images showing a characteristic glaze. Moreover, the firmness of the fixation is generally inadequate. For example, bending, folding, or scratching of a picture image causes it to have a smear by stretching or to come off the substrate, the problems incidental to pressure fixation remaining as heretofore.
U.S. Pat. No. 4,442,194 specifies a capsule toner of the so-called liquid filling type, which features a filling consisting of a solvent, a resin for fixation dissolved in said solvent, and a pigment as a colorant. A capsule toner of this type is shown in FIG. 5.
When a capsule toner of the liquid filling type is used, the shell (3) is broken by pressure, allowing the resin for fixation in the form of solution therein to spill out and cover the pigment (2) as the resin is fixed to a substrate. Generally a capsule toner of the liquid filling type produces a picture image with satisfactory results in quality, the picture image showing no glaze as would be seen in one of a solid filling type capsule toner.
Various problems remain to be solved, however. If it is attempted to lower the pressure for the fixation, inadequate firmness of the fixation arises as a problem. Even when the pressure is sufficient to break the toner shells, the capsule toner may be driven into the interspace between fibres of the transfer sheet if its surface is coarse, so there is a problem that the toner left out of fixation attaches to and smears the paper. Other problems include inadequate cleaning and filming on the surface of a photosensitive member.
Generally, a capsule toner containing a liquid in the shell is obtained more or less as a true globe in shape, having a relatively small surface area. This small surface area of the toner and inadequate catch between the toner and the carrier raise a problem of low charge even when a toner has been charged by friction with a carrier and one of unsatisfactory rising characteristic in charging that tends to cause the toner to fly about.
Another problem poses itself when a spherical toner is subjected to a running test in a PPC copying machine using a blade cleaning method. The toner is not completely cleaned away from the photosensitive member but part of it attaches to the photosensitive member by fusion as it clears the blade cleaner. Charging takes place on this toner which has attached by fusion, and is developed in the following copying procedure, and eventually produces black streaks in the picture image.